Winter
Leader: Night's King
UA: Winter is coming. Every tile owned by Winter will be a snow or ice tile. Can only recruit White Walkers, only one for each city owned. Does not pay unit maintenance. Units die if not on a snow or ice tile. Units can move accros ice.
UU: White Walker. Replaces Warrior. If an enemy unit dies while within ten tiles of a White Walker it will revive and join Winter.
UB: White Walker Altar. Replaces Shrine. Can only be build in the capital. After you end your turn every tile bordering a snow or ice connected to your capital shall be covered by snow or ice.
This would be a fun faction as they would literally be spreading Winter accros the map. This while constantly growing a larger army and breaking the enemy economy with the snow.